Jan 24 (Reuters) - Spark Therapeutics Inc, whose breakthrough gene therapy to treat a rare form of blindness was approved by U.S. regulators in December, said on Wednesday that it had licensed rights to the drug outside the United States to Switzerland-based Novartis AG.

Philadelphia-based Spark said it will keep U.S. rights to Luxturna, or voretigene neparvovec, which is the first approved gene therapy for an inherited disease. It plans to launch the one-time treatment in March, charging an unprecedented $850,000.

Spark will receive $105 million from Novartis and up to another $65 million in milestone payments based on near-term European regulatory approval and initial sales in certain markets. Spark is also entitled to future royalties on net sales outside the United States.

In a statement, Spark said the deal leverages Novartis' "large, existing commercial and medical infrastructure in ophthalmology, as well as its commitment to commercializing genetic-based medicines."

Novartis, which sells opthalmology drugs including macular degeneration treatment Lucentis, last year won U.S. approval for Kymriah, the first of a new type of potent gene-modifying immunotherapy for leukemia.

"We think this deal is a positive for Spark, as the company will be able to focus on the U.S. launch of Luxturna, leaving the ex-U.S. launch and commercialization to Novartis, a leader in the ophthalmology space," Jefferies analyst Michael Yee said in a note to investors.

Luxturna treats inherited retinal disease caused by defects in a gene known as RPE65, which affects between 1,000 and 2,000 people in the United States. It works by delivering by an eye injection viral vector particles containing a correct copy of the gene to retinal cells, restoring their ability to make a needed enzyme.

Shares of Spark, which fell 2 percent to close at $55.36 in regular trading on Wednesday, were up 3 percent at $57 after hours. The shares have lost nearly 25 percent of their value since mid-December when investors were disappointed by trial data for Spark's experimental gene therapy for hemophilia A. (Reporting y Deena Beasley; Editing by Susan Thomas and Tom Brown)